The present disclosure relates to a radio receiver for receiving a transmitted signal.
As radio receivers, superheterodyne receivers are generally used. Superheterodyne receivers convert a received signal to an intermediate frequency (IF) signal having a given frequency at all times. Hence, this type of receivers has the following advantages among others: an interference signal is easily attenuated (interference less occurs) because the pass band of a filter can be fixed; an amplifier circuit can be easily made up; and the reception sensitivity is high. Also, under the condition where an especially strong interference signal exists, the capability of eliminating an interference signal can be improved by using a filter whose band is variable at the time of processing of an IF signal.
FIG. 9A is a diagrammatic view showing a mixer and the frequencies of its input/output signals. FIGS. 9B and 9C are views illustrating examples of the relationship between the frequency of a desired signal and the frequency of an image signal.
For reception of amplitude modulation (AM) broadcasts and frequency modulation (FM) broadcasts that use a higher-frequency signal than AM broadcasts with a common circuit, an oscillation signal from a voltage-controlled oscillator (VCO) is generally frequency-divided before being inputted into a mixer circuit for converting a received signal to an IF signal. Such frequency-divided oscillation signal is herein called a local oscillation signal. As shown in FIGS. 9B and 9C, when either of the relationships expressed by two equations:fvco=fd+fif  (1)fvco=fd−fif  (2)is established among the frequency fd of a desired signal reception of which is desired, the frequency fif of an IF signal and the frequency fvco of a local oscillation signal (local oscillation frequency), an IF signal having the same frequency will be obtained. The case of equation (1) (FIG. 9B) is called upper heterodyne and the case of equation (2) (FIG. 9C) is called lower heterodyne.
In relation to the above, a signal (called an image signal) having a frequency fim satisfyingfim=fvco+fif  (3)fim=fvco−fif  (4)for equation (1) and equation (2), respectively, also obtains an IF signal having the same frequency as the IF signal for the desired signal.
Existence of such an image signal causes interference (called image interference) during reception of the desired signal, worsening the reception performance of the receiver for the desired signal. It is therefore required to remove the effect of the image signal in some way or another. To secure the reception performance in an area badly affected by the interference of the image signal, the receiver should desirably have an image rejection ratio of 80 to 90 dB.
For rejection of an image signal, a radio receiver using a mixer having an image signal rejection function is well known. However, the image rejection ratio of such a radio receiver is of the order of 40 to 50 dB. Also known is a method in which a band-pass filter that allows a desired signal to pass but attenuates an image signal is inserted upstream of a mixer. Such a band-pass filter however does not serve as a useful means for a radio receiver that gives an IF signal having a frequency of several hundreds of kHz because it fails to sufficiently attenuate an image signal.
Under the above circumstances, receivers using a signal having a frequency that is less likely to cause image interference have come to be considered. An example of such receivers is disclosed in WO2001/048909. This receiver measures the image signal strength for the cases of FIGS. 9B and 9C, and adopts a local oscillation signal with which the image signal strength is lower to thereby be less affected by an image signal.
However, when the image signal strength is high in both the cases of FIGS. 9B and 9C, the receiver described above fails to satisfy a desired image rejection ratio and thus fails to receive a desired signal without interference.
The frequency difference between the image signals in the cases of FIGS. 9B and 9C is equivalent to 4×fif. If 500 kHz is selected as the frequency of the IF signal, the frequency difference between the image signals is 2 MHz. In this case, therefore, the strength comparison will be made between signals 2 MHz apart in frequency. In the band of FM broadcasts, there are a myriad of combinations of two signals 2 MHz apart in frequency, and the possibility that the two signals are both high in strength is not low.